Astronomers have spotted a faint, previously unknown star whipping around Sagittarius A*, the Milky Way’s supermassive black hole, on the tightest and most extreme orbit ever recorded.
This sequence of images, taken with the GRAVITY instrument at ESO’s Very Large Telescope Interferometer, show several stars orbiting Sagittarius A*. One of these stars, known as S301, was recently found to pass much closer to the black hole than any other known star. Image credit: ESO / GRAVITY Collaboration.
“Sagittarius A*, the closest massive black hole in the Galactic center at a distance of 8,300 parsecs,” said Dr. Karim Abd El Dayem from the Observatoire de Paris and colleagues.
“Its vicinity has allowed measuring individual stellar orbits around it.”
“The stars act as test particles and probe the gravitational potential around the 4.3-million-solar-mass black hole.”
The newly-discovered star, designated S301, was spotted in the spring of 2023 using the GRAVITY instrument on ESO’s Very Large Telescope Interferometer.
By tracing its motion forward and combing back through older data, they confirmed sightings stretching back to 2017.
S301 completes a full orbit in just 8.7 years — shorter than any previously known orbital period around Sagittarius A* — and follows an extraordinarily elongated path, swinging in as close as roughly 136 to 142 times the black hole’s Schwarzschild radius at its closest approach.
At this point, the star reaches blistering speeds of about 25,000 km/sec, more than 8% of the speed of light.
Based on its faintness, the astronomers infer that S301 is a compact, Sun-like main-sequence star with roughly 1.1 to 1.5 times the mass of the Sun — far too small and dim to be one of the giant stars previously seen near the black hole.
They suggest S301 was likely once part of a tight binary star system that wandered too close to Sagittarius A* and was ripped apart by the black hole’s gravity, a process known as the Hills mechanism.
One star would have been flung outward as a hypervelocity star, while its companion, S301, was captured onto its current tight, elongated orbit.
Because S301 comes so close to Sagittarius A*, it is the first star known that could be used to directly measure the rotation of a black hole.
According to Einstein’s general theory of relativity, a spinning black hole drags spacetime along with it and twists it, which impacts the orbits of surrounding stars.
The effect is felt more strongly for objects orbiting fast-rotating black holes at close range.
